The invention relates to a surface sensor, comprising a frequency-selective surface with periodically arranged THz structures sensitive to terahertz radiation (THz radiation), in particular THz resonator structures each having an associated polarization axis. The invention further relates to a method of producing a surface sensor. The invention moreover relates to a system having a surface sensor, as well as a use of the surface sensor.
Periodic antenna structures in the microwave range or other electromagnetically active periodic structures are known, for example, from US 2007/011431 A1 or US 2006/0152430 A1. A THz measurement unit equipped with a single THz structure for molecular analysis is known from DE 102 57 225 B3.
A surface sensor of the type cited above is usually formed on a substrate, the surface of which is provided with periodically arranged THz structures, i.e. structures which are sensitive to THz radiation. Usually, these structures are configured as THz resonators which are sensitive to emitting and/or detecting THz radiation in a specific resonant range. A frequency-selective surface with symmetrical THz resonators is for example known from the article by O'Hara et al., “Thin-film sensing with planar terahertz metamaterials: sensitivity and limitations” in OPTICS EXPRESS Vol. 16, No. 3, pages 1786 et seq. (Feb. 4, 2008).
Further publications on the general background of THz technology are:    P. H. Siegel, “Terahertz technology in biology and medicine”, IEEE Trans. Microwave Theory Tech. 52, 2438 (2004).    E. R. Brown, J. E. Bjarnason, T. L. J. Chan, A. W. M. Lee, and M. A. Cells, “Optical attenuation signatures of Bacillus subtillis in the THz region”, Appl. Phys. Lett. 84, 3438-3440 (2004).    D. L. Woolard, E. R. Brown, M. Pepper, and M. Kemp, “Terahertz frequency sensing an imaging: A time of reckoning future applications?”, Proc. IEEE 93, 1722-1743 (2005).    J. Barber, D. E. Hooks, D. J. Funk, R. D. Averitt, A. J. Taylor, and D. Babikov, “Temperature-dependent far-infrared spectra of single crystals of high explosives using terahertz time-domain spectroscopy”, J. Phys. Chem. A 109, 3501 (2005).    J. Chen, Y. Chen, H. Zhao, G. J. Bastiaans, and X.-C. Zhang, “Absorption coefficients of selected explosives and related compounds in the range of 0.1-2.8 THz”, Opt. Express 19, 12060 (2007).    B. M. Fischer, M. Walther, and P. Uhd Jepsen, “Far-infrared vibrational modes of DNA components studied by terahertz time-domain spectroscopy”, Phys. Med. Biol. 47, 3807-3814 (2002).    J. Zhang, and D. Grischkowsky, “Waveguide terahertz time-domain spectroscopy of nanometer water layers”, Opt. Lett. 29, 1617 (2004).    M. Nagel, P. Haring-Bolivar, M. Brucherseifer, H. Kurz, A. Bosserhoff, and R. Büttner, “Integrated planar terahertz resonators for femtomolar sensitivity label-free detection of DNA hybridization”, Appl. Opt. 41, 2074 (2002).    M. Nagel, F. Richter, P. Haring-Bolivar, and H. Kurz, “A functionalized THz sensor for marker-free DNA analysis”, Phys. Med. Biol. 48, 3625 (2003).    C. K. Tiang, J. Cunningham, C. Wood, I. C. Hunter, and A. G. Davies, “Electromagnetic simulation of terahertz frequency range filters for genetic sensing”, J. Appl. Phys. 100, 066105-1-3 (2006).    T. Baras, T. Kleine-Ostmann, and M. Koch, “On-chip THz detection of biomaterials: a numerical study”, J. Biol. Phys. 29, 187 (2003).    M. Bruchseifer, M. Nagel, P. Haring-Bolivar, H. Kurz, A. Bosserhoff, and R. Büttner, “Label-free probing of the binding state of DNA by time-domain terahertz sensing”, Appl. Phys. Lett. 77, 4049 (2000).    T. Driscoll, G. O. Andreev, D. N. Basov, S. Palit, S. Y. Cho, N. M. Jokrest, and D. R. Smith, “Tuned permeability in terahertz split-ring resonators for devices and sensors”, Appl. Phys. Lett. 91, 062511 (2007).    C. Debus and P. Haring-Bolivar, “Frequency selective surfaces for high sensitivity terahertz sensing”, Appl. Phys. Lett. 91, 184102 (2007).    M. Kafesaki, Th. Koschny, R. S. Penciu, T. F. Gundogdu, E. N. Economou, and C. M. Soukoulis, “Left-handed metamaterials: detailed numerical studies of the transmission properties”, J. Opt. A: Pure Appl. Opt. 7, p. 12 (2005).    A. K. Azad, J. Dai, and W. Zhang, “Transmission properties of terahertz pulses through sub-wavelength double split-ring resonators”, Opt. Lett. 31, 634 (2006).    D. Grischkowsky, S. Keiding, M. van Exter, and Ch. Fattinger, “Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors”, J. Opt. Soc. Am. B 7, 2006 (1990).    W. H. Padilla, A. J. Taylor, C. Highstrete, Mark Lee, and R. D. Averitt, “Dynamical electric and magnetic metamaterial response at terahertz frequencies”, Phys. Rev. Lett. 96, 107401 (2006).    J. P. Pendry, A. J. Holden, D. J. Robbins, and W. J. Stewart, “Magnetism from conductors and enhanced nonlinear phenomena”, IEEE Trans. Microwave Theory Tech. 47, 2075 (1999).    J. D. Baena, J. Bonache, F. Martin, R. Marqués Sillero, F. Falcone, T. Lopetegi, M. A. G. Laso, J. García-García, I. Gil, M. F. Portillo, and M. Sorolla, “Equivalent-circuit models for split-ring resonators and complementary split-ring resonators coupled to planar transmission lines”, IEEE Trans. Microwave Theory Tech. 53, 1451 (2005).    J. F. O'Hara, E. Smirnova, A. K. Azad, H.-T. Chen, and A. J. Taylor, “Effects of microstructure variations on macroscopic terahertz metafilm properties”, Active and Passive Electronic Components 2007, 49691 (2007).    J. A. Defeijter, J. Benjamins, F. A. Veer, “Ellipsometry as a tool to study adsorption behavior of synthetic and biopolymers at air-water-interface”, Biopolymers 17, 1759-1772 (1978).    Markelz, S. Whitmire, and J. Hillebrecht et al., “THz time-domain spectroscopy of biomolecular conformational modes”, Phys. Med. Biol. 47, 3797-3805 (2002).    B. M. Fischer, M. Hoffmann, H. Helm, et al., “Terahertz time-domain spectroscopy and imaging of artificial RNA”, Opt. Express 13, 5205-5215 (2005).    A. K. Azad, A. J. Taylor, E. Smirnova, and J. F. O'Hara, “Characterization and analysis of terahertz metamaterials based on rectangular split-ring-resonators”, Appl. Phys. Lett. 92, 011119 (2008).    H.-T- Chen, W. J. Padilla, J. M. O. Zide, S. R. Bank, A. C. Gossard, A. J. Taylor, and R. D. Averitt, “Ultrafast optical switching of terahertz metamaterials fabricated on ErAs/GaAs nanoisland superlattices”, Opt. Lett. 32, 1620-1622 (2007).    M. A. Cooper, Drug Discovery Today 11, 1061 (2006).    SRU Biosystems, Inc., www.srubiosystems.com    Biacore Life Sciences, www.biacore.com    G. U. Lee, L. A. Chrisey, E. E. O'Ferrall, D. E. Pilloff, N. H. Turner, R. J. Colton, Israel J. Chem. 36, 81-87 (1996).
Biomolecules such as DNA, proteins and the like are known to have binding-specific properties in the THz frequency range. THz resonators can be used to read out a property at the highest possible sensitivity. A surface with periodically arranged resonators can be read out in a particularly easy manner.
Such an arrangement is known under the term frequency-selective surface (FSS) as initially defined. An FSS, as a rule, is comprised of metallic resonator structures. Examples of resonator structures with symmetrically constructed TH resonators are known for example from the articles by Yoshida, “Terahertz sensing method for protein detection using a thin metallic mesh”, APPLIED PHYSICS LETTERS 91, 253901 (2007) and Driscoll et al., “Tuned permeability in terahertz split-ring resonators for devices and sensors”, APPLIED PHYSICS LETTERS 91, 062511 (2007). An FFS possesses frequency-dependent transmission and reflection properties tailored to the respective application, and they are used for example as reflectors in antenna systems, or else utilized for radar camouflage, e.g. of combat aircraft or the like.
The simplest resonator structure is a wire dipole having a length of λ/2. The equivalent electric circuit diagram of such a resonator is a resonant LC circuit. If this element is resonantly excited by an external field, a current oscillating at a resonance frequency fr will flow through the wire. A surface with periodically arranged wire dipoles or dipoles of other configurations hence virtually acts in resonance like a closed metallic surface and shows a maximum reflection—in case of zero loss R(fr)=1 and T(fr)=0. In this case, the resonance frequency of a single element formed from a THz structure, respectively a THz resonator, deviates from that of the entire array due to coupling effects. The typical FSS applications as a rule require the flattest possible “gap-free” frequency-response curve. The FSS application for detecting e.g. biological samples having a low material volume is relatively new. The approach is based on the fundamental property of resonant structures to locally “store” the excitation energy for a certain period of time, and thus to enable a strongly increased interaction with the sample material as compared to simple transmission. The sensor response to a deposited material consists in a shift of one or more resonance frequencies of the FSS.
Actual numerical simulations such as those in the article by C. Debus et al., “Frequency selective surfaces for high sensitivity terahertz sensing”, APPLIED PHYSICS LETTERS 91, 184102 (2007), have shown that the frequency anomaly occurring between two adjacent interfering resonances in the form of a zero in the reflectance spectrum responds particularly sensitive to the slightest changes in the dielectric environment. A simple way of generating such a frequency anomaly is to break the symmetry within the unit cell of the FSS. Asymmetrically split-ring resonators (aSRR) such as in the article of Debus et al. cited above, likewise take advantage of this effect.
Basically, split-ring resonators are for example known from other fields of application of electromagnetic resonators, such as e.g. from US 2007/0114431 or JP 64001304A.
Apart from this, there are a plurality of approaches for marker-free biomolecule detection. Although extremely interesting in the economical and technical respect, none of these methods have hitherto been able to prevail over the established label-based method. Marker-free detection methods inter alia exist on the basis of:                optical surface plasmons,        resistive techniques,        mechanical sensors,        acoustical wave sensors,        optically sampled nanoparticle sensors.        
Each of the above-mentioned methods has its own advantages and disadvantages. A varying weighting, however, can establish that the previous methods mentioned above have one or more deficiencies in the following fields: sensitivity, cost efficiency, compactness, probe rate, ease of operation, measurement accuracy, fault tolerance.
FSS as a whole have shown to be an impressive alternative to avoiding the above-mentioned deficiencies. However, the FSS discussed above having asymmetrically split-ring resonators have to date shown a strong dependence on the polarization direction of the incident readout beam. This polarization dependency poses a problem for the technical application, since each misadjustment could erroneously be interpreted as a sensor response—hence there is a high cross-sensitivity. FSS having completely symmetrical resonator elements such as those of the initially cited articles, do not have this polarization dependency when at a sufficiently low enough spacing, but in this case also lack the necessary resonance indifference required to achieve a sufficiently high enough sensor sensitivity.
It would be desirable to realize a sufficiently high enough sensor sensitivity along with polarization independence.